I. Field
This disclosure relates to an optical transmitter, and particularly to a dense wavelength division multiplexing transmitter architecture including a multi-wavelength emitting laser, optics and electronics for outputting data.
II. Background
Conventional optical transmitters contain a single wavelength light generator, a light modulator, and circuitry for accepting xe2x80x98client dataxe2x80x99 and applying that data to the light modulator so that the light is modulated according to the data. Typically the modulator allows transmission of the light to represent a xe2x80x9c1xe2x80x9d and blocks transmission of the light to represent a xe2x80x9c0xe2x80x9d. The transmitters also include various control and monitoring means. To create a conventional Dense Wavelength Division Multiplexing (DWDM) system, a multiplicity of these transmitters, one for each wavelength, may be assembled and the output of each combined using a multiplexer and placed on a single optical fiber. The specific wavelengths to be used are specified by the International Telecommunications Union and are commonly referred to as the xe2x80x9cITU Gridxe2x80x9d.
The wavelength of these lasers must be maintained to high precision as narrow band filters in the demultiplexer will be used to separate the wavelengths at the receiving end of the fiber. Unfortunately, the wavelength of conventional DFB lasers may drift with aging and temperature. Most detrimental is when two adjacent wavelengths drift toward each other. This requires filters in the demultiplexer which can unambiguously separate wavelengths separated by the small amount corresponding to the worst case drift. Filter expense increases as the separation between adjacent wavelengths decreases.
A further challenge to optical communications systems based on DWDM is ensuring that multiple wavelengths have equal relative power. This is desired to assure adequate Signal-to-Noise ratio in each channel without overloading the receiver of any one channel. Furthermore, optical communications systems often have one or more optical amplifiers between the transmitter and the receiver, and amplifiers used in these systems typically have the attribute of increasing the difference in power between wavelength channels.
The process of causing all the wavelengths to have the same power is referred to herein as equalization. In DWDM systems, equalization may be used in conjunction with optical amplifiers in order to compensate for the fact that the gain of these amplifiers is not the same at all wavelengths. In, for example, Erbium Doped Fiber Amplifiers (EDFA), gain varies only slowly with wavelength. Consequently, equalization can be achieved by introducing a filter whose loss also varies slowly and proportionally to the amplifier gain can also be employed.